The present invention relates generally to electric or hybrid electric vehicle propulsion systems. More specifically, the present invention relates to the design of electric traction motors or machines for use in electric or hybrid vehicles.
In today""s automotive market, there exists a variety of electric propulsion or drive technologies used to power vehicles. The technologies include electric traction motors such as DC motors, AC induction motors, switched reluctance motors, synchronous reluctance motors, brushless DC motors and corresponding power electronics. Brushless DC motors are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to DC motors and AC induction motors. Brushless DC motors typically operate with a permanent magnet rotor. A permanent magnet rotor may be configured as a surface mount or interior or buried permanent magnet rotor. An interior permanent magnet (IPM) motor or machine has performance attributes, when compared to DC motors and AC induction motors, that include relatively high efficiency, relatively high torque, relatively high power densities, and a long constant power operating range which make an IPM machine attractive for vehicle propulsion applications.
Permanent magnets buried inside a rotor for a brushless DC motor exhibit high reluctance directly along the magnetic axis or the d-axis due to the low permeability of the permanent magnets. While along the q-axis, between the magnetic poles or magnet barriers of an IPM rotor, there exists no magnetic barrier, and reluctivity to magnetic flux is very low. This variation of the reluctance around the rotor creates saliency in the rotor structure of an IPM machine. Therefore, the IPM rotors have reluctance torque in addition to the permanent magnet torque generated by the magnets buried inside the rotor. Reluctance in the d-axis can be created by one magnet such as found in a single barrier rotor design.
A single magnet of the one barrier rotor design can also be split into several layers creating a multi-barrier design. The multi-barrier design reduces leakage and improves the rotor saliency. Accordingly, motors having multi-barrier rotors have numerous performance advantages over a single barrier rotor design, including relatively high overall efficiency, extended high speed constant power operating range, and improved power factor. Improved saliency of the multi-barrier rotor helps to lower the amount of magnets or magnetic material in an IPM machine, as compared to a single barrier IPM machine or surface mounted permanent magnet machine, by reducing dependency on magnetic torque. The amount of magnetic material needed to generate a specific torque and wattage rating depends on the level of saliency of the rotor. The higher the rotor saliency, the lower the magnet material usage for the same overall machine performance. Electric motors having a multi-barrier rotor design, as compared to single barrier design, generate higher rotor saliency.
The reduction of magnetic material in an electric motor rotor is desirable from a cost standpoint. Lower amounts of magnetic material usage also alleviate some of the problems that are encountered in permanent magnet machines, such as fault problems and also spin loss problems due to the magnetic field generated by rotor magnets that is present even if the motor is not producing any torque. A pure synchronous reluctance motor that has similar rotor geometry to the multi-barrier permanent magnet (PM) design but no magnetic material in the rotor is a relatively low performance machine. The multi-barrier IPM electric motors have the beneficial attributes of both the synchronous reluctance machine and the permanent magnet machine and are excellent candidates for vehicle propulsion. The major difficulty involved with IPM machines is the design and manufacture of the rotor.
Magnets in an IPM machine can be pre-magnetized and then inserted inside the rotor. This magnet insertion is a complex and relatively costly step that adds manufacturing steps to the assembly of the IPM machine.
Post-magnetization of inserted magnetic material is possible if the magnets are inserted near the rotor surface, as shown in FIG. 2. For post-magnetization, magnetic material may be preformed outside of the rotor, inserted into the rotor, and then magnetized. This is usually the case with sintered magnets, which require a certain orientation. A further type of magnetic material used that may be used in an IPM rotor is bonded magnets, which are usually mixed with a plastic, such as PPS, and may also be preformed outside of the rotor and then inserted into the rotor. However, generally bonded magnetic material is injected into the rotor cavities under high temperature and pressure.
Electric motors having multi-layer buried magnets in their rotors, as shown in FIG. 4, exhibit excellent performance characteristics for vehicle propulsion application. The problems associated post-magnetizing such a rotor geometry would result in a large amount of magnetic material buried deep within the rotor that may only partially magnetize or not magnetize at all, resulting in a waste of material. Post-magnetization works efficiently for only magnetic material buried or located near the surface of the rotor as shown in FIG. 2. For magnetic material buried relatively deep in the rotor, post-magnetization is difficult due to the weakening of the magnetizing field.
The present invention includes a method and apparatus for the design of an IPM machine rotor. The present invention removes magnetic material from the regions of the rotor which cannot be effectively or strongly magnetized during the post-magnetization process. For instance, the entire outer barrier of the rotor of FIG. 4 can be easily magnetized. However, the middle section of the inner regions of the rotor may not be exposed to a magnetic field strong enough to fully magnetize these regions. In the present invention, magnetic material is removed from these areas and may be kept void of any material. This ensures the high reluctance to the magnet axis or the d-axis needed to produce the saliency in the rotor.
In alternative embodiments of the present invention, the middle regions of an IPM rotor may be filled with non-magnetic thermal material to allow rotor heat to escape through the motor shaft area, improving the rotor thermal performance. Keeping the middle region of a rotor void of any material or inserting non-magnetic thermal material does not change the rotor saliency or the reluctance torque. However, removing magnetic material from certain regions of the rotor will reduce the magnetic field and, as a consequence, the torque produced by this magnetic field and the interaction of the stator current will be reduced. To compensate for the loss of magnetic field and the corresponding magnetic torque, the magnetic field strength of the remaining magnetic material is increased.
The strength of a magnet is typically defined by the magnetic energy product (MEP). MEP is proportional to the product of magnetic remnant flux density, Br, and the coercivity, Hc. MEP is measured in units of energy per unit volume. When this energy product is multiplied by the total magnetic volume, the amount of MEP or energy from the magnet is obtained. It has been verified by finite element simulation that the magnetic torque remains substantially the same if the MEP times the magnetic material volume is substantially the same in both the geometries shown in FIGS. 4 and 5. Accordingly, the MEP of the magnets of the geometry of FIG. 5 needs to be improved in the same proportion as the magnetic material volume is reduced due to the removal of magnetic material near the center and middle of the rotor. Magnetic material cost is also related to the magnetic energy product and the total magnetic material volume. The relationship is complex and also depends on many other factors such as the chemical composition of the magnetic material, magnetic type, and the type of processing. However, the total magnetic material cost will be reduced without a loss of motor performance if the total magnetic material volume is reduced and the MEP multiplied by the magnetic material volume remaining in the motor rotor is substantially the same.
By reducing the amount of magnetic material used in the IPM rotor of the present invention and keeping the MEP times the magnet volume substantially the same, the cost of the IPM motor may be reduced. Furthermore, this invention would allow complete magnetization of the outer as well as the inner magnetic material or barriers, thus reducing the volume of valuable magnetic material in the rotor and the cost of the motor, as the manufacturing and material cost will be reduced. Furthermore, motor mass will be reduced as a consequence of the reduced magnetic material volume or mass, and thermal performance of the rotor will also be improved, enabling the cooling of the magnetic barriers.